Multiscale Description of Transport and Reaction in Coated Catalyst Layers

Multiscale description of transport and reaction in coated catalyst layers

Vladimír Novák^a, Petr Kočí^a,*, Tomáš Gregor^b,

František Štěpánek^a, Miloš Marek^a

^a Department of Chemical Engineering, Institute of Chemical Technology, Prague, Technická 5, CZ 16628 Prague, Czech Republic

^b New Technologies Research Centre, University of West Bohemia, Univerzitní 8, CZ 30614 Pilsen, Czech Republic

(^*Corresponding Author's E-mail: milos.marek@vscht.cz )

ABSTRACT

The coated catalytic layer In standard 1D+1D models of monolith channel, is usually treated as homogeneous, with transport properties represented by effective diffusivity [1]. This effective diffusivity is typically calculated from random pore model [2] that considers contribution of both small micro/mesopores (intrinsic nanostructure of the used material) and larger macropores (inter-particle structure influenced by the coating procedure). In reality, however, the coated layer may contain additional cracks and cavities (Fig. 1). These structural defects can enhance effective diffusivity in the layer and improve the performance of the catalyst, so that their occurrence is beneficial as long as the adhesion and mechanical durability of the layer is preserved.

In this contribution we apply novel experimental and computational methods to investigate the contribution of cracks and cavities to diffusion in catalytic coatings. A sample of commercial catalytic monolith for automotive exhaust gas aftertreatment is scanned by X-ray computer microtomograph (µCT) to obtain realistic 3D images of the coated layer (Fig. 1). Spatially 3D diffusion is then simulated within a section of the reconstructed porous structure, using a validated methodology [3,4,5,6]. Additional simulations are performed for the model coating without any cracks or cavities (i.e., containing only standard meso- and macro-pores). By comparison of the simulation results, the contribution of cracks and cavities to transport characteristics of the layer is quantified. The evaluated effective diffusivities are compared with random-pore model approximations [2], and further utilized in multi-scale simulations of combined reaction and transport [3] that reveal the impact of transport effect on overall performance of the monolith reactor.

The proposed combination of advanced 3D structural analysis technique and detailed modeling approach significantly extends the level of detail in understanding the internal transport effects in catalytic coatings of monoliths.

   

Fig. 1. 3D reconstruction of catalytic coating in commercial monolith with square channels (CLEERS research community [7] reference commercial catalyst LNT BMW 120i model year 2009) – sections around the channel corner are shown. Monolith substrate is displayed as dark grey, catalytic coating is bright, and black color corresponds to void space (pores, cavities and cracks). The sample scanned by X-ray computer microtomograph (µCT).

Acknowledgements

This work has been financially supported by the Czech Science Foundation (GACR P106/10/1568). The X-ray microtomography scans and 3D reconstruction results were obtained within the CENTEM project CZ.1.05/2.1.00/03.0088, co-funded by the European Research Development Fund as part of the Czech Ministry of Education, Youth and Sports' OP RDI programme. The work was further supported by the project LH 12086 of the Czech Ministry of Education, Youth and Sports.

References

[1] Güthenke A., Chatterjee D., Weibel M., Krutzsch B., Kočí P., Marek M., Nova I., Tronconi E., Current status of modelling lean exhaust gas aftertreatment catalysts, Advances in Chemical Engineering 33 (2007), 103-211.

[2] Wakao N., Smith J.M., Diffusion in catalyst pellets, Chemical Engineering Science 17 (1962), 825-834.

[3] Kočí P., Novák V., Štěpánek F., Marek M., Kubíček M. Multi-scale modelling of reaction and transport in porous catalysts. Chemical Engineering Science 65 (2010), 412-419.

[4] Novák V., Štěpánek F., Kočí P., Marek M. Evaluation of local pore sizes and transport properties in porous catalysts. Chemical Engineering Science 65 (2010), 2352-2360.

[5] Novák V., Kočí P., Marek M., Štěpánek F., Blanco-García P., Jones G. Multi-scale modelling and measurements of diffusion through porous catalytic coatings: An application to exhaust gas oxidation. Catalysis Today 188 (2012), 62-69.

[6] http://www.vscht.cz/monolith

[7] http://www.cleers.org